The majority of suspension systems for vehicles on the road today are passive. These systems utilize multiple linkages, springs and shock absorbers (dampers), which dissipate energy imparted by the wheels to the vehicle body. They play a critical role in the comfort, handling and road holding ability that is provided by a vehicle. One of the key elements of a suspension system is the shock absorber or damper. Conventional dampers are used only to dissipate the kinetic energy that is imparted to the vehicle suspension, for example, by irregularities in the road surface such as bumps and potholes.
A typical damper in a passive system is designed to only dissipate energy. It includes a piston in a cylinder that is rigidly attached to and supported by a longitudinal piston rod. The piston and shaft combination is moveable in both a compression stroke, where the piston moves further into the body of the damper, and an extension or rebound stroke, where the piston moves in the opposite direction. In a conventional suspension system, road induced vertical wheel motion causes the piston within the damper to travel axially relative to and within the cylinder in a manner that forces a viscous fluid, contained in the cylinder, through various restrictive orifices. This process causes the road induced energy in the suspension to be dissipated.
The force on the piston face opposite the shaft (hereinafter referred to as the “front piston face”) is determined by multiplying the fluid pressure with the cross-sectional area of the piston that is perpendicular to the longitudinal axis of the cylinder. However, the force on the piston face on the side which is attached to the shaft (hereinafter referred to as the “rear piston face”) is determined by multiplying the fluid pressure with the annular area determined by the difference in the cross-sectional piston area perpendicular to the longitudinal axis of the cylinder and the cross-sectional area of the shaft.
When operated within design tolerances (i.e. without overheating), the performance of a damper in a passive system is largely fixed and determined by its structure and the physical properties of the viscous fluid and damping valves used. Therefore, comfort, handling and road holding ability offered by a particular vehicle model remains largely unvarying. This limitation has been ameliorated to a limited extent by the introduction of adaptive suspension systems. Adaptive suspensions are still largely passive systems in that they still can only react to road induced forces except that these systems are able to alter system behavior such as, for example, by changing the firmness of the suspension.
Active suspension systems, on the other hand, introduce much greater flexibility and control by utilizing the damper, or shock absorber element, as an actuator, i.e. a forcing (pushing and pulling) and damping (retarding) device. Active suspensions (sometimes referred to as active dampers) are not only capable of damping road induced forces, but also of generating and applying active forces in real time.
MacPherson struts are a specific class of dampers used in many vehicle suspension systems where the damper is arranged between the vehicle body and its wheel assembly. A strut simultaneously acts as a structural member of the suspension system and a damping element. Therefore, it is exposed to significant transverse forces and bending moments. Although use of a strut suspension allows for an economical system, the side loading on the damper piston rod and damper housing necessitates that the piston rod diameter and bearing arrangement be larger than in non-strut damper applications. This results in a larger difference between front and rear piston face areas. Because of this area difference, when the damper is compressed, the volume of oil displaced by the piston cannot be accommodated by the volume made available from behind the piston. On the other hand, when the damper is extended or rebounds, the volume of fluid displaced by the back face of the piston cannot supply sufficient oil to fill the volume opened up by the motion of the front face of the piston. Therefore, conventional strut dampers are normally of the ‘twin tube’ passive or semi-active configurations whereby the excess or deficit in hydraulic fluid is accommodated via a low-pressure reservoir that is in fluid communication with the compression and/or extension chambers via passive or semi-active valving. The compression chamber is defined as the fluid filled volume in front of the piston, while the extension chamber is the fluid filled volume that is behind the piston.
Conventional systems use a low pressure ‘twin-tube’ arrangement where the excess hydraulic fluid is stored in a low pressure reservoir until it is needed. However, active systems typically must utilize high hydraulic pressures in order to provide the required forces in both directions. Consequently, in an active strut system, elevated reservoir pressures are oftentimes needed.
However, the larger piston rod diameter of a strut damper results in a greater mismatch between the front piston face area and the rear piston face area. When the damper is in a neutral state, i.e. not being exposed to longitudinal forces, the front and rear of the piston are typically exposed to reservoir pressure. Therefore, because of the area mismatch, an unbalanced force is applied to the damper piston which would tend to cause the damper to extend, resisted only by the force of the suspension spring and vehicle weight. The situation is made substantially more problematic during use when as the damper temperature increases. As the oil and reservoir gas expands reservoir gas pressure may reach 150 bar (approximately 2200 psi) or more due to thermal expansion.
Active suspension systems apply energy to the suspension in response to various road loads applied to a wheel in order to improve vehicle dynamics, occupant comfort and safety. In order to achieve a desired level of suspension performance, an active suspension system needs to have energy either already present or capable of being provided at an appropriate time. In the case of hydraulic systems, the necessary energy corresponds to a necessary hydraulic pressure and flow rate. A conventional approach used in hydraulic active suspension systems, to ensure that energy is applied in a timely manner, is a continuously operating pump that provides a desired hydraulic pressure and flow.
These types of systems control the fluid flow and pressure provided by continuously operating the pump either by controlling the displacement of the pump and/or using one or more electronically controlled valves to control the fluid flow and pressure from the pump to the actuator. Some systems, especially systems including fixed displacement pumps, use valves to bypass the actuator or fill the compression and rebound chambers. However, it should be noted that in some hydraulic systems, the speed of the pump may be rapidly adjusted to increase or decrease the hydraulic flow and/or pressure. Hydraulic suspension systems are typically powered using a hydraulic actuator associated with a remotely located hydraulic power source used to transfer hydraulic fluid to and from the actuator via an arrangement of hydraulic hoses or tubes. Hydraulic power sources may include various components including, for example, an electric motor and pump assembly as well as a fluid reservoir.